The long-term objective of the proposed research is to understand in detail the mechanism of action of phytochrome on the expression of specific genes. These genes include those encoded by gene families for the small subunit of ribulose 1,5 bisphosphate carboxylase/oxygenase (rbcS genes) and for the major light- harvesting chlorophyll a/b-protein associated with photosystem I (cab genes). The specific objectives for the next five years include: 1. to determine whether the differences in the responses of individual rbcS genes of lemna gibba involves differences in transcription or in post-transcriptional processes, and to study the bases of the observed differences; 2. To determine the basis for the organ specificity of the different rbcS genes in Lemna; 3. To isolate and characterize any trans-acting factors that might be involved in the phytochrome regulated transcription responses and to characterize DNA regions to which they bind; 4. To characterize a gene which we have isolated from Lemna and whose transcription is negatively regulated by phytochrome action; 5. To compare different positively and negatively regulated genes in terms of possible binding factors and/or transduction chain differences that lead to the different gene expression responses to phytochrome action that we found in Lemna; 6. To complete characterization of the phytochrome response of the cab genes of Arabidopsis thaliana; 7. To isolate Arabidopsis mutants in the phytochrome response pathway(s) that govern nuclear transcriptional changes and light-dependent germination; and 8. To analyze the recovered mutants both genetically and biochemically.